The heterogeneous immunoassay exists as a method for assaying hormones and other chemical constituents contained in very small quantities in blood and the like. In this method, a luminescent reaction, such as a chemiluminescence as described in Patent Document 1 or an enzyme-based electrochemiluminescence as described in Patent Document 2, is detected by a photomultiplier tube. A calibration technique for a photomultiplier tube is disclosed in Patent Document 3. In this technique, a spectrophotometer has a plurality of calibration curves associated with a plurality of detection sensitivities of a photomultiplier tube.
In addition, Patent Document 4 discloses a method of signal processing in a detection instrument using a photomultiplier tube in the chemiluminescent method, and Patent Document 5 discloses a method of sensitivity adjustment in a photomultiplier tube.
Quantitative measurements on the concentrations of the chemical substances contained in blood, urine, and other body fluids, such as proteins, lipids, sugar, ions and their constituents, are performed at clinical sites. The clinical examination apparatus includes an automatic analyzer where preparation of an aliquot of a body fluid or other liquid sample; mixture of the aliquot with a reagent; and measurement of the change of a substance contained in the reagent as a result of reaction with the reagent are performed. Such an automatic analyzer is configured so that the processes required for analysis, including mixture of a sample with a reagent and reaction at a constant temperature, are successively performed at given times. Automatic analyzers are configured to undertake mixing the sample and reagents required for analysis, causing reactions at a constant temperature, and various other processes, one after another, within a required time.
Of these automatic analyzers, apparatus that measure the minute quantities of hormones and other chemical constituents contained in blood or other substances employ a heterogeneous immunoassay method, in which the luminescence obtained is measured using a highly sensitive element such as a photomultiplier tube.
If a substance to be subjected to high-sensitivity analysis is very small in biogenic content, the concentration itself of the substance has insignificant effects upon constancy, such that the concentration is measured in a significantly wide range. The content of blood thyroid-stimulating hormone, for example, is required to be detected and measured in a range from 0.001 μIU/ml to 100 μIU/ml, this spread between both being as much as 100,000 times.
At the same time, since the objective of the data obtained varies from one concentration region to another, predetermined specific resolution is demanded for an infinitesimal range, a normal range, and a high-concentration range each.
Reagents with varying sensitivity to one specific substance to be measured are often supplied for those measurement items. However, supplying a special reagent for each of analytical items with a very low frequency of analysis involves a heavy economical burden. Therefore, detectors and reaction systems applicable to higher sensitivity (a range of infinitesimal quantity) have been provided, and for higher-concentration measurement, a necessary reagent has been diluted from several times to several hundreds of times, to increase substantial detection sensitivity.
However, if a substance whose content stays in an infinitesimal range and whose concentration equilibrium has been reached in the presence of other constituents in the serum is diluted with a normal saline solution, signal levels may not change according to the particular dilution rate. In addition, an immunoassay, generally called the sandwich method, is used for a substance whose content lies in a low-concentration region, in particular. In the sandwich method, the linearity of output signals is lost since noise such as the luminescence in the reagent constituents is augmented in comparison with the signals derived from the substance to be measured.
In a high-concentration range, in contrast, the relative quantitative ratio of the reagent constituents to the constituents in the sample decreases, which results in insufficient progress of reactions. In this case, the linearity of output signals is also lost.
Quantitative determination of concentrations in a wider range, therefore, may conveniently use nonlinear calibration curves. The calibration curves used include a spline curve, a curve created by combining an exponent and a logarithm, a polynomial calibration curve created by combining a plurality of lines, and more. A calibration curve that becomes a prototype, that is, a master curve is created using at least five points. The calibration curve is often transformed during actual measurement. One method of transforming the calibration curve uses two typical points to shift the curve in parallel with respect to the position of the minimum signal level. Another method is by dimensionally changing the range of the minimum and maximum signal levels in telescopic form to fit the positions of both signal levels. Input data (X-axis data) of the calibration curve denotes the signal levels detected by the detection unit of the system and calculated by the arithmetic unit of the system, and output data (Y-axis data) represents the concentration of the substance measured.
To conduct such transformation appropriately, the signal levels that become the input of the calibration curve need to stay in a fixed range. For this reason, the entire detection system including the detector is adjusted using a pseudo sample that is prepared using weight or capacity beforehand outside the measuring device. Prior to the adjustment of the detection system, desired signal levels with respect to the pseudo sample are assigned and then the desired signal levels are adjusted by controlling the high voltage applied to the detector to obtain the desired signal levels.
The photomultiplier tube commonly used for such high-sensitivity analysis is one kind of vacuum tube. Details are shown in Non-Patent Document 1. A high voltage of about 1,000 V is applied between a cathode that receives light inside the vacuum tube, and an anode that extracts a final signal. The light with which the cathode surface of the photomultiplier tube has been irradiated is multiplied by utilizing the differential potential between both electrodes to obtain an amplification effect of nearly 106.